ap_finalize_sub_req_protocol() shouldn't send an EOS bucket if

one was already sent.

mod_ext_filter performs some one-time processing when it sees EOS.
When it saw EOS more than once and a subsequent attempt to close a
file failed, it issued the log message described in PR 9644.

PR:      9644


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95595 13f79535-47bb-0310-9956-ffa450edef68